Door closure



L. DAVIDSON Feb. 26, 1952 DOORA cLosuRE 2 SHEETS--SHEET l Filed Octf l?,1947 INVENTOR 0a/s @awk/50 ATTORN EY5 L. DAVIDSON DOOR CLOSUR 2SHEETS--SHEET 2 Filed Oct. 17, 1947 llallll Il Illllllll INVENTQR 0a/sDov/Urol? ATTORNEY Patented el). 26,

UNITED STATES PATENT OFFICE 11 Claims.

This invention relates to an improved building construction for closingan opening such as a doorway in a building that normally has great widthand height and has two or more different heights of the large opening.

This invention further pertains to an improved door construction thatwill close a large opening of at least two heights in a building andalso provides for the ecient storage of the door construction when thedoorway is open.

It is well recognized that there are many and difficult problems inproviding proper door constructions for large openings in buildings suchas aeroplane hangers, dirigible hangers, power houses, and the like.

In various types of doors employed for closing the large openings inbuildings such as aeroplane hangers, and the like, it has heretoforebeen the custom to provide doors having a series of leaves all of thesame height and which leaves are joined together and folded back onthemselves to open the door. When such a doorway is part of a hangar,one of the controlling factors is the height of the fuselage and thetail of the aeroplane.

Inasmuch as the width of the fuselages and of the tails of the aeroplaneare relatively small, there is no need of making all of the doorsections or leaves of a height that is desirable to accomodate theheights of the fuselage and the tail. However, in most all instancesheretofore, the doors made have been of a uniform height based upon thegreatest height of the hangar opening. In other instances, the doorswere built with a separate tail gate at the center, and operatedindependently of the main door. In some instances, the leaves have beenconnected together and the operating mechanism employed to move them hasbeen large and complicated and cumbersome as the doors have been heavyand of very large size. The movements of such doors have caused injuriesto some of the operating personnel and often the doors become stuckbefore they have been moved to give the full opening of the building.Also aeroplanes have been damaged by being nicked and otherwise injuredvwhen the doors are not fully opened. Another great disadvantage is thecost of construction and maintenance of the door parts or leaves whenthey all are built to be of the height of the highest part of the hangaropening, or built with a conventional tail gate.

r Big single piece doors for openings of large buildings of this type,are heavy and are very expensive to build and maintain as they must bedesigned to properly resist external and in ternal wind pressures.

It is a novel feature of this invention to provid a buildingconstruction of relatively .long span with a door opening havingat leasttwo different.

heights for the purpose of accommodating an aeroplane or another productof manufacture that has at least two differentv heights in itssilhouette, and to provide efficient and satisfactory door constructionfor closing or opening these different heights of the opening.

It is a further feature to provide an improved door construction of aplurality of separate units, sections or leaves, some of them being ofdifferent heights, and having them cooperate with each other vtoproperly close the opening of the building, and then to be moved to astorage space to open or clear the opening of the building, but at alltimes to support the leaves properly against wind, storm and weatherstresses and strains.

Another` of the advantages of this invention is the provision ofimproved structure at the main opening so that a higher level of theopening may be further raised to another height to permit the tail ofthe aeroplane to be readily moved into the hangar. The necessary guidingstructure for the top of the leaves ofthe door which closes the completeopening may be raised so as to engage the top of the door leavesprovided for the higher heights of the opening.

One of the particular objects of this invention is to provide a seriesof movable leaves of a door of different heights and construction and toprovide at all times at least a two-point support for each of theleaves, preferably at the top and at the base.

It is also a desire of this invention tc provide a seriesV of floorbearing horizontal sliding door leaves of different heights to close anopening of different heights in an aeroplane hangar, or the like, and tohave the door leaves properly and efficiently supported and movable fromclosing position to storage position and vice versa.

It is recognized in an aeroplane hangar or other large long spanbuilding construction that there is to be a high part or portion,usually at the middle of the building opening. It is further recognizedthat this opening does not need to be of great width and then thebuilding opening is reduced in height to at least one lower height orlevel. Other lower heights may be provided as desired. It is alsorecognized that the door construction must provide for the properclosing of these diierent heights, and for the storing of the leaves ofthe door, and to provide for proper storm and weather resistance by thedoor construction.

In the preferred form of the invention herein, there is provided animproved series of floor bearing horizontal sliding door leaves forclosing the hangar opening, and for storing these door leaves so theytake up only a small amount of space and so that they are protected atleast to some extent from snow blockades and wind, by the buildingitself or by a prepared cover or pocket, or by U cooperating points ofcontacts with their supports and with themselves when their storagespace is or is not under cover.

Another feature of this invention is the provision of improvedsupporting and guiding means at the top and bottom of each door leaf sothat when it is in closure position, it will readily withstand theweather and wind stresses and strains.

By reason of the improved construction herein, the door closurestructure may be provided at much less cost than heretofore. Mld withthe lower height leaves there is better efficiency in handling, and atleast one of the lower height door leaves successfully cooperates Withthe higher leaf or leaves to support and maintain them during openingmovement and during storage, thus always providing at least a twopointsupport of each leaf of the door construction. The two-point supportjust mentioned, is considered as the two points that support the leavesof the door to maintain them vertically. This two-point support isconstituted by one support at the base of the leaves and the othersupport is at the top of the leaves. Each of these point supports isconstituted by two or more engaging elements provided across the leafhorizontally. When the higher doors are being moved from engagement withthe guide frame or canopy at the top, they move into engagement with theguiding means on one of the lower leaves of the door. These guidingmeans are located at points below the top of the higher door leaf, butthere is always a two-point support horizontally of the higher doorleaves.

Other features and advantages of the preferred construction of theinvention herein will be noted when referring to the drawings attachedhereto and to the detailed description following. In the drawings,

Fig. 1 is a front view of a building having a large continuous openingof different heights.

Fig. 2 is a sectional view taken on line 2--2 of Fig. 1 of a series ofleaves forming a door for closing the continuous opening of thebuilding, and illustrates a door pocket for receiving the leaves (shownin dotted formation) Fig. 3 is a vertical sectional view taken on theline 3 3 of Fig l in varying scale of one of the doors constructed tocover the greater height of the door opening and illustrates the doorleaves which close the lower heights of the door opening;

Fig. 4 is a sectional View taken on the line 4 4 of Fig. l andillustrates the cooperating means between the door leaves which closethe lower and higher heights of the door opening of the building;

Fig. 5 is a plan sectional view taken on the line 5-5 of Fig. 1illustrating a portion of the construction in Fig. 4 where thecooperating elements between the high and low leaves of the door forclosing the continuous opening engage each other to give support andguidance;

Figure 6 is a sectional view taken on the line 6-6 of Fig. l showing thelower heightr of the various leaves of the door construction and showingthe means of maintaining the lower height door` leaves in their verticalpositions and showing the cooperating means between one of the lowerdoor leaves and one of the higher door leaves shown in dotted lines;

Fig. '7 is a sectional view taken on line 'I-T of Fig. 1 through thedoor pocket and shows top portions. of several leaves of the doorclosure asi A they are compacted into the door pocket for storagepurposes, and

Fig. 8 is a modified front view of part of the structure shown in Fig. 1wherein the main opening in the building may be modified in the futureto give different .aeroplane tail gate heights. This figure also shows asingle height door for closing the future higher portions of theopening.

Referring now to these drawings, Fig. 1 illustrates the front of abuilding, preferably a hanger for aeroplanes, dirigibles, or for powerhouses and the like. The building is shown with a continuous dooropening of at least two different heights. In addition, storage pocketsare shown adjacent the edges of the sides of the building for receivingthe various sections, elements or leaves of a door construction forclosing the continuous opening.

It has been found that it is advisable and efcient to include a seriesof leaves constituting a door for closing the opening and to have theseleaves preferably positioned vertically and supported at points at ornear the bases and at or near the tops thereofso as to withstand windand storm pressures.

In the preferred construction herein disclosed, it is desired to provideguides, or guide frames or guide apparatus or canopy I, of suitablestructure and design at the tops of the varying heights of thecontinuous openingin the building. In the illustration in Fig. 1, theguide frames l are provided for engaging the tops of the leaves of thedoor construction of the lower of the two heights of the opening shownin this disclosure of the invention. It will be understood that theopening of the building may be of several heights. Guide frames orcanopy 2 are provided for engaging the top, or tops, of the leaves whichclose a higher level of the opening. This higher opening may be closedby one or more of the leaves of the door construction. In this instance,however, two door leaves are shown. When desired, one leaf may beprovided to cover the higher level opening. 'I'he number of leavesdecided upon for this purpose is usually determined by the width andheight of opening. It is preferred that the sizes of the leaves shall besuchY that they are easily handled and are of relatively small cost.

In this description, the door leaves for closing the higher and loweropenings are 'noted as A, B, C, and D, respectively, extending from thecenter of the opening to the left. A complementary set of door leavesextend from the center of the opening to the right.

It will be noticed in this preferred construction that the continuousdoor opening has only two height levels, and preferably the higher oneof these is located in the center of the opening, but for specialpurposes it could be located in other positions. When this long spannedbuilding is used as a hanger for aeroplanes, or dirigibles, the higheropening permits the aeroplanes or dirigibles to be moved into thebuilding without being nicked, scraped or damaged. The narrow width ofthe opening is provided to accommodate the narrower fuselage and tailrudder of an aeroplane, but the greater height of opening is necessary.The wings will pass through the lower height opening of the building.

The preferred form of supporting the higher leaf at two p oints or attwo levels to maintain it in vertical position, is to provide a guide 3on one of the leaves of lower height, especially a. leaf that isadjacent, as leaf B, and to provide wheels engaging the sides of thetracks Ill.

an engaging member on leaf A taking the form of bracket 4 and roller 5for cooperating with the guide 3. It is preferred that there shallalways be across the tops of the leaves of the door, a two-point supporthorizontally so that any one of the leaves of the door is alwayssupported against any twisting about its vertical axis. In respect tothe one or more higher leaves, it is desirable to have at least two ofthe supports at its top, and when the leaf is moved from engagement withits guide frame or canopy 2, there will be an engagement of the lowerroller 5 with guides 3 on leaf B so that the higher leaf will alwayshave two points across its width that are in engagement with properguiding and supporting means, thereby eliminating any possibility of thehigher leaf twisting about its vertical axis. When the leaf A has beenmoved so that it registers or corresponds with the ad jacent leaf B, itsguide rollers 5 located between its base and the top will engage theguide 3 carried by the adjacent leaf B.

The lower leaves are provided near their tops with rollers. It ispreferred that these rollers be located at the top of the leaves B, Cand D where they engage drop guides 6 Figs. 3 and 4 which usually are ofmetal carried by the guide frames I and 2.

In moving the leaves of the door construction to storage position, ithas been found helpful in snow country to provide a storage pocket 'Iinto which the leaves may be moved or pushed. It is desirable to have atleast one of these storage pockets adjacent each end of the continuousopening, as illustrated in Fig. l. In Fig. 2 there is ilustrated indotted line how these leaves are readily stored in therpockets 1. It ispreferred in this construction to employ a member 8 attached to the leafD at right angles to the rear edge thereof so that when the leaves ofthe door construction are pulled out of pocket 1 for closing thecontinuous opening in the building, the member 8 will close the front ofthe storage pocket, thus preventing the pocket from becoming filled withsnow and preventing other material from moving therein. It is preferredthat in any of these formed storage pockets that a suitable support,such as frame 9 Fig. 7, be provided for supporting the drop guides 6 sothat the leaves of the door will be maintained in their uprightpositions. A partial view of the construction of the preferred storagepocket is shown in Fig. 7.

In preparing a 4proper type of opening closure, it is desired to provideeach one of the leaves of the door construction so that the leaves willbe slidable horizontally and be licor bearing on rails or tracks Imounted on concrete base I I, thereby providing ready means for movingthe leaves of the door back and forth from closing position. When thehigher leaves A are in closure positions and engage the drop guides 6,carried by guide frame 2 at the higher level opening, the guide frame 2is supported by a suitable construction including an I-beam I2, asindicated in Figs. 1, 3 and 8. It will be understood that any other typeof satisfactory substantial construction and design may be employed.Also noted in Fig, 3 is the preferred type of means of maintaining theseleaves in position on tracks I0 by including the anged wheels I3 withthe depending sides of the It will be seen from the reference to Fig. 3that each of the leaves of the door is supported at its base and at itstop so that it is resistant to wind and weather pressure. lThe rollersI3` and the drop guides 6 cooperate to maintain the leaves in uprightpositions against wind and storm pressures. Other types of buildingstructures for the long spanned building are illustrated in Fig. 6 forsupporting the guide frames I and providing suficient strength tomaintain the leaves against the wind and storm pressures. The top partof Figs. 3 and 4 also show additional building structural elements forgiving added strength and stability.

Referring now particularly to Figs. 4 and 6, it will be understood thatany suitable engaging members may be carried by the leaf A and engagesuitable guides on one of the other leaves, preferably a lower leaf andpreferably adjacent to the leaf A, vbut it Will be understood that thecooperating and engaging portions of the leaves A and B will be suchthat as leaf A is moved out of its guide frame 2 that the lower roller 5willl engage guide 3 on the adjacent leaf B and thereby be supported andmaintained Substantially vertical during the movement of all of the doorleaves into the storage pocket 1. In Fig. 6 the support 4 is shown `asbeing wellattached to leaf A, as indicated in dotted lines in this gure.

The guide 3 carried on door leaf B may be of any desired construction solong as it will receive vlower rollers 5 carried below the top of leaf Aand cooperate with those rollers to maintain the leaf A in itssubstantial vertical position. The preferred construction of guide 3 isthat of a channel iron properly fastened to leaf B. The lower rollers 5on leaf A will roll between the edges of such a channel and maintainleaf A in its substantially vertical position after leaving its topguide 2. When desired, guide 3 may be made of two angle irons as notedin Figs. 4 and '7, or an angle iron fastened in a manner shown in Fig.3.

When the leaves `are moved to storage position and no storage pocket isprovided, the guide frame I may be extended so as to receive and hold atype of supporting frame equivalent to frame 9 that will provide dropguides -6 so that the leaves may be properly supported adjacent eachother and will cooperate with each other as a unit to prevent the windand storm pressures from moving the leaves sidewise.

It is preferred that the leaves of the door shall have a two-pointengagement for maintaining the leaves substantially vertical. Thetwopoint engagement may have one point at the base and the other pointof support above the first point of support, and preferably the secondpoint of support will be somewhere near the top. It will also beunderstood that there may be two points of engagement across ahorizontal of any one leaf so as to prevent that leaf from twisting.Such a two-point horizontal engagement may be provided -by supportingrollers at the base, or by two types of rollers somewhere' between thetop of a door leaf and its base. However, from practical consideration,it is preferable to have two points of engagement both vertically andhorizontally.

In the preferred structure here shown, one point is at the base of adoor leaf and the other point is at a different level, usually at thetop of the particular door leaf. In addition to this twopoint support,there may be two other points of supports on a horizontal level witheach of the vertical or different level supports. These two differentlevel supports may be, as already noted, at the top and bottom of anydoor leaf or the upper or second level may be somewhere below the topVof a door leaf, and somewhere down to ward the base of the door leaf.'I'his type of twolevel support would apply to door leaves of the sameheights, or to a series of door leaves, some of which are of differentheights than others.

In Fig. 8, there is illustrated a type of construction where it ispossible to lower the top guide frame or canopy 2 to a position where itwill engage the top of the door leaf A, which door leaf will be of aheight somewhere between that of the lower door leaves and a height lessthan shown by the position of canopy 2 of Fig. '7. A desired height forfuture use is shown in dotted lines. However, provision is made so thatthe tail gate opening covered -by guide frame 2 may be raised topositions of different heights to accommodate the height of the tail ofone of the larger aeroplanes. These additional heights are considered asthe tail gate openings, so as to accommodate dierent heights of tails onan aeroplane. It is understood that the higher` door leaves may beemployed for the proper heights to accommodate the products to be movedinto the building.

It will be noted, generally, that each of the leaves is supported, atleast, at two different level points at all times, preferably at top andbottom, and thus are readily maintained in substantially verticalpositions. The higher leaves are supported at two points, top andbottom, each when in positions of closing the opening of the building,and these two points are the most efficient places to resist wind andstorm pressures. These higher leaves when moved to storage positions arestill supported at two different level points but the upper support isnot as high as the next lower height of the opening in the building.When in storage position higher leaves are protected or cooperate withthe other leaves closely positioned to give a compact assembly.

It will be understood that various modifications and changes may be madein the preferred form of the invention herein and such modifications andchanges are to be understood as part of this invention, as outlined inthe following claims.

The invention claimed is:

1. In combination, a building having a continuous opening of differentheights, and a plurality of leaves formingv a door for closing saidopening, said leaves being of different heights and when in position ofclosing said opening being supported at different height levels tomaintain them in substantially vertical positions and as so supported tohave them resistant to wind and storm pressures, a support near the baseof each leaf, and supports carried by said building at different heightlevels for engaging the several leaves at point levels above said firstpoint of support for assisting and maintaining said leaves in theirsubstantially vertical positions.

2. In combination, a building having a continuous opening o varyingheights, a plurality of leaves forming a door for closing said opening,at least one of said leaves being of a height to close a higher part ofsaid opening, the other leaves being of lesser heights, rails forsupporting said leaves at the bases thereof and allowing said leaves tobe moved along said rails to storage positions, guide structure carriedby said building for engaging each of said leaves at a point above saidrails for supporting the tops of said leaves, an engaging member mountedon said higher leaf at a height less than the heights of said lowerleaves, a guide mounted on the leaf adjacent said higher leaf forreceiving said engaging member of said higher leaf for supporting saidhigher leaf in substantially vertical position when it is moved out ofits position of closing the higher part of said opening.

3. In combination, a building having a continuous opening of diiferentheights, a plurality of leaves forming a door for closing said opening,two of said leaves constructed to close the higher parts of saidopening, supports at the bases of said leaves, guide structure carriedby said building and being positioned relatively near to the tops ofsaid leaves and engaging said leaves, said supports and said guidestructure maintaining said leaves substantially vertical when inyposition of closing said opening and causing said leaves to resist windand storm pressure, a series of said leaves being of lower height andcooperating with said higher leaves for closing the remainder of saidopening, engaging members mounted on said higher leaves at points belowthe tops thereof, guides mounted on the leaves adjacent the higherleaves at points slightly below the tops of the lower leaves, saidengaging members on each of the higher leaves engaging said guides onthe adjacent leaves when said leaves are moved to storage positionsthereby freeing said opening in said building.

4. In combination, a relatively long span building having a continuousopening of a relatively low height and of a relatively high height, thelatter height being substantially at the middle of the long span of saidbuilding, a series of leaves comprising a door for closing said open--ing, at least one of said leaves constructed to close said higher partof said opening, supports at the bases of said leaves, guides carried bysaid building and engaging the tops of said leaves to cooperate withsaid base 'supports to maintain said leaves in substantially verticalpositions and thereby to have said leaves resist wind and stormpressures, said leaves being movable to storage positions to free saidopening, an engaging member mounted on said higher leaf at a point belowits top, a guide mounted on a lower leaf in position to cooperate withsaid engaging member on said higher leaf when the same had been movedout of its closing position thereby to support said higher leaf at twopoints when in storage and thereby maintaining the same in substantiallyvertical position.

. 5. In combination, a building having a continuous opening of differentheights, a plurality of leaves forming a door for closing said opening,two of said leaves constructed to close the higher parts of saidopening, supports at the bases of said leaves for holding said leavesand for having the same move over said supports, guide structure carriedby said building at positions near said leaves and engaging the tops ofsaid leaves, said supports and said structure maintaining said leavessubstantially vertical when in position of closing said opening andcausing said leaves to resist wind and storm pressures, a series of saidleaves being of lower height and cooperating withsaid higher leaves forclosing the remainder of said opening, engaging members mounted on saidhigher leaves at points below the tops thereof, guides mounted on theleaves adjacent the higher leaves at points slightly below the tops ofsaid lower leaves, said engaging members on each of said higher leavesengaging said guides on the adjacent leaves when said higher leaves aremoved to storage positions thereby freeing said opening in saidbuilding, a door pocket for receiving said leaves in storage,

9 and drop guides for maintaining the same in substantially verticalposition therein.

6. In combination, a relatively long span building having a continuousopening, said opening being of at least two different heights, a seriesof leaves forming a door for closing both heights of said opening,tracks for supporting each of the leaves at their bases, said tracksextending from the positions of closing said opening for each leaf to astorage position adjacent the sides of said opening, track engagingmeans carried by the bases of each leaf and cooperating with said tracksin overhanging manner to prevent side movement of said leaves, guidemembers carried by the building and positioned at the tops of saidleaves for engaging said leaves to maintain them in substantiallyvertical position and for making said leaves resistant to wind and stormforces, guide frames for said higher leaves extending only over the topof said higher opening, guide frame for said lower leaves, engagingmembers on said higher leaves, guides on the lower leaves for engagingsaid engaging members to maintain said higher leaves in substantiallyupright position when said leaves have been moved to storage positionand away from said guide frames at the top of said higher opening.

7. In combination, a building of long span having a continuous openingof varying heights, two series of leaves forming doors for closing saidopening, engaging means for supporting each series of said leaves atdifferent height levels, storage pockets positioned adjacent the ends ofsaid continuous opening for receiving said leaves when moved to clearsaid opening, and a member carried by the rear edge of the leaf havingthe shortest travel into said pocket from the position of closing saidstorage pocket opening, said member being held at right angles to saidleaf at the rear edge thereof said member attached to the last leafhaving a width for substantially closing an opening in said storagepocket when said leaves are in position of closing the opening in saidbuilding.

8. In combination, a hangar of wide span and having a continuous openingfor the admission of an aeroplane into said hangar, said opening bein-gof two heights, one height being slightly greater than the height of thewings and the other height being slightly greater than the height of thefuselage and tail rudder, a series of leaves forming a door for closingsaid continuous opening, at least one of said leaves being of a heightto close the higher opening, other of said leaves being of heights toclose the remaining portion of said opening, rails for supporting thebases of said leaves, pulley type rollers mounted in the bases of saidleaves and engaging the top and sides of said rails to allow said leavesto move horizontally along said rails and to prevent said leaves frommoving sidewise of said rails, guide frames carried by the buildingstructure at the top heights of said opening, rollers mounted on thetops of said higher leaves and positioned to engage said guide frames tomaintain said leaves in substantially vertical position, second guideframes carried by the building structure at the top heights of saidlower height leaves and for engaging said lower height leaves, a guidemounted on the side of the lower leaf adjacent said higher leaf, anengaging member mounted on the side of said higher leaf for engaging thesaid guide on the adjacent leaf and for supporting said higher leaf whenthe top thereof has been moved from said rst guide frame positioned atthe top 10 of the higher portion of the opening of said buildmg.

9. A horizontally slidable floor bearing door construction for closingan opening of varying heights in a building, said door construction consisting of a plurality of leaves cooperating to close said opening, oneof said leaves being higher than another leaf and in its position ofclosing a portion of said opening is supported at its top and base toresist movement out of its substantially vertical position, iioor tracksfor slidably supporting said leaves at their bases, guide structurecarried by said building and engaging the top of said higher leaf, aroller supporting frame mounted on the side of said higher leaf at apoint lower than the lower height of said opening, a guide membermounted on said lower height adjacent leaf for receiving said rollerattached to said higher leaf when said higher leaf has been moved out ofits closure position and toward a storage position, said roller and saidguide member cooperating to maintain said higher leaf in substantiallyvertical position While said leaves are being moved to the storageposition.

10. A horizontally slidable floor bearing door construction for closingan opening of varying heights in a building, said door constructionconsisting of a plurality of leaves positioned upright and slidable todoor closing position or to storage position, tracks for supporting eachof said leaves and extending to a storage position, flanged wheelscarried by said leaves at their bases for engaging the tops and sides ofsaid tracks to resist side movement of said leaves, guide frames mountedon the structure of said building at the top of the various heights ofsaid opening for engaging the tops of said leaves to maintain them insubstantially upright position and to thereby assist in resisting windand storm pressures, the length of said guide frame over the higherportion of said opening being substantially of the width of said higheropening, cooperating roller bearing engaging member mounted on thehigher leaf at a point below the top thereof, guides on one of the loweradjacent leaves for cooperating with said roller of said engaging memberfor supporting said higher leaf substantially upright when said leaf hasbeen moved from position of engaging said guide frames at their tops.

11. A horizontally slidable floor bearing door construction for closingan opening of Varying heights in a building, said door constructionconsisting of a plurality of leaves, floor tracks for supporting saidleaves, overhead guides attached to said building at the tops of saidopenings to engage the tops of said leaves and for supporting saidleaves substantially upright, one of said leaves being of greater heightthan another of said leaves, and a guide on the leaf adjacent saidhigher leaf for engaging said higher leaf and supporting the same afterit has been moved from the higher part of said opening of said building.

LOUIS DAVIDSON.

REFERENCES CITED The following references are-of record in the file ofthis patent:

UNITED STATES PATENTS Bassett Apr. 21, 1942

